Bipolar diffusion charging of aerosol particles—II. Influence of the concentration ratio of positive and negative ions on the charge distribution

Various mathematical solutions to the convective-diffusion equation for noninteracting Brownian particles were carried out to predict deposition of submicrometer particles onto a flat surface in viscous, three-dimensional (axisymmetric) stagnation-point flow at clean-room velocities ( -100 cm/s). The particle deposition aspects modeled included electrostatics, inasmuch as both difision and electrostatics are the dominant mechanisms expected. The results were obtained in terms of dimensionless groups for deposition, convective-diffusion, and electrostatic attraction. It was found that the deposition velocity can be well approximated by a simple combination of the convective-diffusion velocity and the eletrostatic velocity. These results are translated into practical terms, examples are given, and the predictions are compared with predictions made by other methods. A disk 20 cm in diameter charged to 2000-V potential is shown to attract a particle 0.1 pm in diameter so as to produce an electrostatic deposition velocity 180 times larger than the diffusion deposition velocity, giving it a deposition velocity nearly equal to that of a particle 10 pm in diameter settling under gravity.

Section: Aerosol Charge Distributionmentioning

confidence: 98%

Predicted Deposition of Submicrometer Particles Due to Diffusion and Electrostatics in Viscous Axisymmetric Stagnation-Point Flow

Cooper¹,

Peters²,

Miller³

1989

“…This charge distribution is obviously much different from that shown in Figure 2 and illustrates that for real situations where aerosols are neutralized in a region of high ionization, we should expect the charge distribution to be asymmetric. See also Porstendorfer et al (1984).…”

Section: High Ionization Rate-asymmetric Chargingmentioning

confidence: 99%

The Nonequilibrium Character of the Aerosol Charge Distributions Produced by Neutralizes

Hoppel

Frick

1990

The mechanisms that control the polar ion concentrations downstream of an ionized region are examined and it is shown that the ratio of positive to negative ion concentrations is not constant. The imbalance in the ionic concentrations caused by unequal diffusion of ions to the walls and to aerosol particles is magnified in the ion ratio as ionic recombination rapidly depletes ions of both polarities equally. Consequently, the aerosol charge distribution is not in equilibrium but is evolving in response to the changing ion environment. The conclusions drawn are supported by numerical modeling and by measurements of ionic concentrations and ratios of negatively to positively charged particles downstream of the ionized region. Several existing neutralizers are evaluated and a prototype ionizer which produces an aerosol with a nearly symmetric equilibrium charge distribution is discussed.

“…Moon (1984) measured the charge distribution of diesel exhaust particles by a combination of a differential mobility analyzer (DMA) and an electrical aerosol analyzer (EAA) but the resolution of the EAA limits the power of this method. Another approach combining a DMA and an optical aerosol spectrometer was used (Porstendorfer et al 1984) but the particle sizes were relatively large (0.5-2 µm) due to limitations of the optical aerosol spectrometer. Integral techniques (Forsyth et al 1998) have been used but they cannot distinguish the charge polarity.…”

Section: Introductionmentioning

confidence: 99%

Bipolar Diffusion Charging of Aggregates

Xiao

Swanson

Pui

et al. 2012

In this paper, the effect of particle morphology on bipolar diffusion charging is studied. A modified tandem differential mobility analyzer (TDMA) method used to measure the charge distribution of submicron particles in the range of 70-300 nm is described in detail. The method requires an independent measurement of the neutral fraction, followed by the measurement of the size-dependent charge distribution, which requires the knowledge of the neutral fraction. The method was validated experimentally using dioctyl sebacate and ammonium sulfate spherical particles and compared with Fuchs' theory. Diesel particles and silver aggregates were used to evaluate the impact of morphology on charging. The results show that aggregates have a slightly lower (about 7%) neutral fraction than spheres. These results are in agreement with the predictions of Lall and Friedlander's theory and previous studies. However, results from charge distribution indicate that more (about 46%) particles are negatively charged than predicted by Lall and Friedlander's theory, while 32% fewer particles are positively charged. This relatively large asymmetry between the negative and the positive charge fraction is not fully predicted by either Fuchs' or aggregate charging theories. Our results suggest that the current inversion method of scanning mobility particle sizer (SMPS) data, based on Fuchs' or Lall and Friedlander's distribution, would underestimate the total number concentration by about 15% or 27% if applied to diesel aggregates.